In xerography, or electrophotographic printing/copying, an electrophotographic imaging member can be electrostatically charged. For optimal image production, the electrophotographic imaging member can be uniformly charged across its entire surface. The electrophotographic imaging member can then be exposed to a light pattern of an input image to selectively discharge the surface of the electrophotographic imaging member in accordance with the image. The resulting pattern of charged and discharged areas on the electrophotographic imaging member can form an electrostatic charge pattern (i.e., a latent image) conforming to the input image. The latent image can be developed by contacting it with finely divided electrostatically-attractable powder called toner. Toner can be held on the image areas by electrostatic force. The toner image can then be transferred to a substrate or support member, and the image can then be affixed to the substrate or support member by a fusing process to form a permanent image on the substrate or support member. After transfer, excess toner left on the electrophotographic imaging member can be cleaned from the surface, and residual charge can be erased from the electrophotographic imaging member.
Electrophotographic imaging members can be provided in a number of forms. For example, an electrophotographic imaging member can be a homogeneous layer of a single material, for example vitreous selenium, or it can be a composite layer containing an electrophotographic layer and another material. In addition, the electrophotographic imaging member can be layered.
Conventional layered electrophotographic imaging members can generally have at least a flexible substrate support layer and two active layers. These active layers can generally include a charge generation layer that can contain a light absorbing material, and a charge transport layer that can contain charge transport molecules. These layers can be in any order, and sometimes can be combined in a single or a mixed layer. The flexible substrate support layer can be formed of a conductive material. Alternatively, a conductive layer can be formed on top of a nonconductive flexible substrate support layer.
Conventional electrophotographic imaging members can be either a function-separation type photoreceptor, in which a layer containing a charge generation substance (charge generation layer) and a layer containing a charge transport substance (charge transport layer) can be separately provided, or a monolayer type photoreceptor in which both the charge generation layer and the charge transport layer can be contained in the same layer.
Conventional electrophotographic imaging members can have an undercoat layer interposed between the conductive support and the charge generation layer. Examples of conventional undercoat layers are disclosed in U.S. Pat. Nos. 4,265,990; 4,921,769; 5,958,638; 5,958,638; 6,132,912; 6,287,737; and 6,444,386; incorporated herein by reference in their entireties.
Thick undercoat layers can be desirable for electrophotographic imaging members because thick undercoat layers can provide longer life spans, can provide resistance to carbon fiber, and can permit the use of less expensive substrates. However, “ghosting” can be a problem when using thick undercoat layers.
In particular, when an image is formed on a photoreceptor, it can be the result of some areas being exposed and others not. The exposed areas can have a different surface potential than the areas that have not been exposed. In theory, once a section of the photoreceptor goes through the erase portion of the cycle, all portions of that section should have the same surface potential. This ensures that upon charging, the surface potential can be the same.
With ghosting, it is believed that the erase does not bring the surface potentials to the same value. Upon charging, the previously exposed and unexposed areas can have a different potential and even when they see the same amount of exposure with the next cycle, some areas can be darker or lighter than others. Those areas that can be lighter or darker can be the latent images from the previous cycles.
Negative ghosting refers to the latent image printing lighter than the surrounding areas, while positive ghosting refers to the latent image printing darker than the surrounding areas.
What is needed is a binder that can improve the ghosting properties and/or improve performance of thick undercoat layers and electrophotographic imaging members containing thick undercoat layers.